In PC-online, the frequency band that is not used so far in CATV broadcasting, 5 to 50 MHz band for instance, is frequency-division multiplexing and time-division multiplexing to utilize in two-way data transmission. TV broadcasting and data transmission are carried out in the same cable. Consequently, in an upstream line transmitting data from each household PC to a base station, without precise control of signal frequency, signal level or timing, a problem may be caused on transmission quality or the like of other data transmission or TV broadcasting.
Other problems, such as signal level variation or distortion of the signal in the cable, may occur due to an impedance mismatch. The output impedance may vary based upon whether data is transmitted. In order to avoid this, problem output impedance of each household PC should be kept at a constant value. In addition, since output terminals of several hundred sets of PCs are always connected to the cables, summation this is a of all noises caused therefrom, that is, streamed noise becomes a problem. FIG. 5 shows a RF output device for use with an upstream line of an existing PC-online.
In FIG. 5, first, in an up-converter 1, signal a modulated based on transmission data, for instance, QPSK (Quarternary Phase Shift Keying) is input in one end and a carrier from an oscillator 2 is input in the other end, followed by up-conversion of the input modulated signal to a prescribed frequency. Then, with an RF switch 3 on/off the signal is carried out and with a selection filter 4 due to such as SAW (surface acoustic wave) excellent in selectivity unnecessary frequency component is removed. At a down-converter 6 in which the output signal from the selection filter 4 is input as one input and output of a VCO (Voltage Controlled Oscillator) 5 is input as the other input, of the down converter 6 the frequency of the output signal from the selection filter 4 is shifted to a prescribed frequency. Here, the VCO 5 is frequency-controlled by a PLL (Phase-Locked Loop) circuit 7 and can output arbitrary frequencies due to control from the PC.
An image frequency component of the down-converter 6 output is removed of the down-converter 6 of the down converter 6 output by an image filter 8, and after adjustment of the signal level by a GCA (Gain Control Amplifier) 9, is output with a prescribed output impedance.
According to an RF output device thus constituted, since on/off switching of signal is carried out by an RF switch 3 disposed at an interval where frequency is converted for signal selection, direct current level is not influenced due to the on/off switching signal. Accordingly, with a simple diode switch, this function can be materialized. In addition, there is merit that since regardless of on/off of signal circuit operation conditions after the down-converter 6 are constant, the output impedance also becomes constant.
However, the on/off switching signal is carried out at a relatively earlier stage that is remote from the final output terminal and furthermore signal amplification is implemented at a stage after the RF switch 3. Consequently, there is a disadvantage that even when the RF switch 3 is turned oft, output noise becomes large due to the noise caused by circuits after the RF switch 3.
This problem can be solved by moving the RF switch 3 to the later stage where signal level is larger. However, a diode switch that will be considered in general has a disadvantage that it can not handle such a large signal. Moreover, it causes a variation in the direct current level due to the on/off switching signal. This variation in the direct the current level causes a change in operation conditions in the later stage which results in a variation in output impedance.
In addition, if the VCO 5 is kept operating during the off state of the RF switch 3, there is a problem of carrier leakage which due to off-setting of the down-converter 6, output of the VCO 5 is output as it is from the down-converter 6. The carrier leakage can not be completely eliminated by the following image filter 8. As a result of this, there is a disadvantage that since the carrier leakage component is equivalent with noise, output noise becomes large.
Accordingly, as shown in FIG. 5, the VCO 5 is required to be furnished with a switch that is not originally required, thereby oscillation being stopped when the RF switch 3 is turned off by switch signal SW.